[codex] Surface runtime warnings in codex exec (#27415)

## Why

`codex exec` drops thread-scoped warning notifications. Warnings
discovered while a thread starts, including unreadable or invalid UTF-8
project `AGENTS.md` files, therefore become silent.

## What changed

- Process global and primary-thread warning notifications while
continuing to ignore warnings from unrelated threads.
- Render runtime warnings in human output and expose them through the
existing non-fatal error item in JSONL output.
- Add focused routing, rendering, and malformed project-instruction
coverage.

codex-rs/exec/src/lib.rs

@@ -1256,6 +1256,11 @@ fn should_process_notification(
 ) -> bool {
     match notification {
         ServerNotification::ConfigWarning(_) | ServerNotification::DeprecationNotice(_) => true,
+        // TODO(anp) resolve duplicate startup warnings
+        ServerNotification::Warning(notification) => notification
+            .thread_id
+            .as_deref()
+            .is_none_or(|candidate| candidate == thread_id),
         ServerNotification::Error(notification) => {
             notification.thread_id == thread_id && notification.turn_id == turn_id
         }